Hydraulic fracturing is a well stimulation technique which involves injecting a fracturing fluid into the formation at rates and pressures sufficient to rupture the formation or widen compressed potential flow conduits, i.e., fissures, cracks, natural fractures, faults, lineaments and bedding planes. In most formations, the earth stresses are such that a vertical crack or fracture is also formed by the hydraulic fracturing treatment. In certain types of formations, the small flow conduits which exist naturally are widened under the hydraulic fracturing process. Once the artificially created fracture is initiated, continued injection of the fracturing fluid causes the hydraulic fracture to grow in length, height and width. A particulate propping agent suspended in a pressurized carrier fluid is then introduced into the relatively larger fractures to maintain them in a propped condition when the fracture-inducing pressure is subsequently relieved. The type and size(s) of the propping agents have been selected based on their ability to prop open the large fractures created in the formation.
In the fracturing of most formations, it is desirable to optimize the width, length and height of the propped fracture in order to increase fracture conductivity. It is known that the success of the well stimulation is strongly influenced by the geometry of the propped fracture. As the fracture width increases, increased fracture lengths can improve well stimulation.
The width of the fracture is normally obtained by controlling variables, such as fluid viscosity and injection rate to achieve the desired fracture geometry. Although large dynamic widths are frequently obtained, the width of the closed fracture is substantially less than the dynamic width, mainly because of the relatively low concentration of proppant in the carrier fluid. In other words, most of the volume in the carrier fluid is liquid, which leaks off into the formation through small flow conduits leaving the proppant wedged in the larger fractures in the formation walls, but unable to enter the small flow conduits.
The improvement in injectivity or productivity of a well by fracturing the formation depends directly on the retained conductivity of a propped fracture system. A wide variety of different techniques and propping agents have been disclosed in the prior art. The following U.S. patents are illustrative of the prior art methods and materials.
For example, U.S. Pat. No. 2,879,847 discloses geometrical shapes that can be introduced into the proppant-fracture system to improve permeability. Protrusions on the sides of three-dimensional objects, such as spheres or the like serve to increase permeability between the spheres or other shapes.
U.S. Pat. No. 3,235,007 discloses multiple layers of respective proppants including metals, ceramics, plastics, steel shot, aluminum, glass-beads and crushed and rounded walnut shells, peach pits, coconut and pecan shells.
U.S. Pat. No. 3,417,819 discloses the use of glass beads flowed into a fracture system with a high viscosity liquid during fracturing.
U.S. Pat. No. 3,701,383 describes electroless metal plating followed by a proppant displaced into the fracture system.
U.S. Pat. No. 3,780,807 discloses injection of a fluid suspension of coarse particles with fine grains of sand or other material bonded to the outer surfaces to maintain pathways between the particles.
U.S. Pat. No. 3,976,138 discloses injection of an alumina propping agent of at least 30 mesh size introduced into a fracture system in a multi-layer distribution scheme.
U.S. Pat. No. 4,029,148 utilizes color-coded proppant that are particles injected at different depths so that the source of the proppant particles can be determined in the event that they backflow and are recovered during later production.
U.S. Pat. No. 4,157,116 discloses the use of material injected to plug a zone around a wellbore in a subterranean formation.
An analysis of the teachings of the prior art technical and patent literature reveals that the methods of formation stimulation have been directed to maintaining flow passageways between particles for retaining permeability, while at the same time maintaining particle contact for retaining high structural strength for propping the fracture open. This approach tends to allow closure over time of production or injection of fluid because of the small movement of the particles with a resultant infusion of particle edges and the like into the passageways to restrict permeability. Moreover, these various types of particles and methods have been resistant to creating the desirable layering in a fully packed propping system in a fracture. Further, these methods have tended to fail in deeper formations where the pressures tending to close a fracture were even greater.
It is known that the flow capacity of certain high compressive strength, deep, over-pressured reservoirs varies with net confining pressure. This net confining pressure is the result of the difference in the overburden pressure and the reservoir pore pressure. As this reservoir pressure is decreased with hydrocarbon depletion, the net confining pressure increases, reducing flow capacity in the matrix and also in small flow conduits.
Many deep, high temperature, low permeability, high compressive stress or over-pressured carbonate and sandstone subterranean formations bearing hydrocarbons contain small flow conduits in the form of, e.g., natural fissures, cracks, natural fractures, lineaments, bedding planes and faults. Productivity from such subterranean formations is determined in large measure by the contribution of hydrocarbons passing through these small flow conduits and into the wellbore. However, the fluid loss (leakoff) that occurs during hydraulic stimulation of these subterranean formations is also increased due to the presence of these small flow conduits.
Although the importance of fracture width has long been recognized by those working in the art, propped widths larger than about 0.3 inches are normally not achieved in deep reservoirs. The contribution of the naturally occurring small flow conduits has been ignored by the prior art fracturing methods, and this despite the recognition of the contribution of such small flow conduits to fluid loss by leakoff during fracturing.
Proppants are of three types: sand, resin coated sand and ceramic proppants. Propping agents, or proppants, include naturally occurring sand, man-made intermediate ceramics, high-strength ceramics, sintered bauxite and resin coated (deformable) sand.
Intermediate strength proppants are defined by reference to the operating conditions into which the proppants will be introduced, i.e., intermediate stresses and temperatures. For example, an ISP will be selected for use at closure stress that is between 4,000 and 8,000 psi and at a bottom hole static temperature of up to 375° F.
High strength proppants would include sintered bauxite. Sintered bauxite, a type of ceramic proppant with a high alumina content, low silica and low clay content, is the strongest proppant available and is used at the greatest depths.
The use of intermediate strength proppants (ISP) and high strength proppants (HSP) have proven effective in enhancing hydrocarbon flow through the induced hydraulic fracture. Conventional fracturing of subterranean formations utilizing quartz sand or other propping agents is ineffective in the small flow conduits of these high compressive stress formations.
Accordingly, it is the object of this invention to provide a method of propping small flow conduits that obviates the disadvantages of the prior art and provides a natural flow system that resists collapse and closure and that retains permeability through a novel mechanism not heretofore employed.
Another object of the invention is to provide an improved propping method for use in deep, high compression formations that will substantially improve hydrocarbon production.
Yet another object of the invention is to provide a novel propping method that can be used advantageously with both acid fracturing and hydraulic fracturing techniques in various types of reservoir rock formations.